In spite of continuous advances in the level of monolithic integration of power functions, a discrete power semiconductor co-packaged with one or more integrated circuits to perform the functions of drive, protection and control provides in many applications a more economical and a better performance solution than a totally integrated one.
This is particularly true when the function to be integrated requires blocking voltages and current levels that are at the limits of the state of the art. Voltage requirements close to the limits of the state of the art add complexity to the process, while large currents require large silicon areas. Either of these requirements has a significant impact on the cost of the integrated solution.
Co-packaging a power semiconductor die with a separate analog and/or digital integrated circuit is an alternative solution that allows more flexibility in the choice of the type of device and process. By way of example, the power device can be implemented with a vertical geometry and specific process techniques that maximize ruggedness, reliability and silicon utilization. The signal processing integrated circuit, on the other hand, could be implemented in low voltage, high density technologies aiming at low cost or high performance or speedy customization or other appealing feature. As a result, cost and performance of the combination can be closely optimized for the specific application. Monolithic implementation cannot provide this level of flexibility and versatility with the available technology.
One major disadvantage of the co-packaged solution is the fact that, by separating the control and protection functions from the power device, one key parameter of its operation, namely temperature, is not immediately available to the control and protection circuitry. A simple and fundamental operation, like overtemperature protection of the power device, cannot be accurately and economically performed if the temperature sensing circuitry is mounted at some distance from the power device or, as it is frequently the case, if they are mounted on a common substrate with relatively high thermal resistance.
As shown later, under the narrow assumption of steady state operating conditions, the overtemperature protection circuitry on the analog integrated circuit could determine the temperature on the power device if the thermal resistance of the two dice between themselves and to the ambient is known, together with the instantaneous power dissipated into the power device itself and the temperature of the analog integrated circuit.
However, overtemperature protection is most useful under fault or other transient conditions, which cause rapid temperature rises. Under these conditions, the knowledge of the parameters mentioned in the previous paragraph is not sufficient to establish the instantaneous temperature of the power device because of the thermal capacitances present.